This invention relates to electrostatographic reproduction machines, and more particularly to such a machine including an automatically self-aligning charging device assembly.
Generally, the process of electrostatographic copying is executed by first using a corona generating or charging device to uniformly charge a photoreceptive member, and then exposing a light image of an original document, positioned in registration on a platen, onto the charged photoreceptive member. Exposing the charged photoreceptive member to a light image discharges the photoconductive surface thereof in areas corresponding to non-image areas in the original document, while maintaining the charge on other areas thereof, particularly on the image areas, thus creating an electrostatic latent image on the photoreceptive member. The undischarged areas including the electrostatic latent image are subsequently developed with charged toner particles into a visible toner image. The toner image is thereafter transferred from the photoreceptive member onto a copy sheet on which the image is then fused or permanently affixed in order to provide a hard copy reproduction of the original document.
The described process is well known and is useful for light lens copying from an original, as well as for printing documents from electronically generated or stored originals. Analogous processes also exist in other electrostatographic applications such as, for example, digital printing applications wherein the latent image is generated by a modulated laser beam or ionographic printing and reproduction, where charge is selectively deposited on a charge retentive surface in accordance with an image stored in electronic form.
In the any of the above processes, a conventional form of a corona generating or charging device that can be used is disclosed, for example, in U.S. Pat. No. 2,836,725 as a basic corotron device. As disclosed, the corotron device includes a conductive corona generating electrode in the form of an elongated wire that is partially surrounded by a conductive shield. The corona generating electrode, or so called coronode, is provided with a DC voltage, while the conductive shield is usually electrically grounded and the dielectric surface to be charged is spaced proximate to the wire. Alternatively, the corotron device may be biased in a manner taught, for example, in U.S. Pat. Nos. 2,879,395, and 5,324,941 and 5,324,942 which describe a type of charging device known as a scorotron. In a scorotron, an AC corona generating potential is applied to the conductive wire electrode while a DC biasing potential is applied to a conductive shield partially surrounding the electrode. This DC potential regulates the flow of ions from the electrode to the surface to be charged so that the charge rate can be adjusted, making this biasing system ideal for self-regulating systems.
Another type of a charging device that is usable in any of the above processes is a pin array device that includes a charging electrode which may be an electrically conductive strip having projections, or scalloped portions in the form of teeth members. The projections or teeth members are integrally formed with, and extend from, a longitudinal edge of the electrode. This type of charging device is disclosed for example in U.S. Pat. No. 3,691,373 to Compton et al. As shown, this type of corona generating device may further include a screen and/or an auxiliary electrode as well as various additional conductive shields for regulating charging current to control the uniformity of produced charge.
Regardless of the particular type, a charging device when used in an electrostatographic machine, is mounted spaced from, and axially relative to the photoreceptive member of the machine. In order not to produce hard copies of images on sheets with toned or black borders, non-image border areas on the photoreceptive member must either not be charged by the charging device, or if charged must then be erased or discharged prior to development with charged toner particles. "Charging and then erasing or discharging" such border areas after uniform charging involves additional use of discharge devices, and is therefore relatively more costly. In addition, it is also relatively more risky in that more or less border area than is necessary may be erased, thus resulting in poor quality images. On the other hand, the relatively less risky and less expensive alternative which is "not to charge such border areas in the first place", requires a precise alignment of an edge of the areas to be charged (which is the same as a position where the charging device starts charging) with a document registration position or edge on a fixed platen.
Unfortunately, however, due to the effects of manufacturing and assembly tolerances, achieving such precise alignment normally requires skills not possessed by an ordinary machine user or customer. This can be more of a problem in the case of charging devices that are manufactured and provided along with other components as customer replaceable (cartridge) units or CRU's that are to be mounted in the machine by the user or customer. In such cases, the-start-of-charging position of the charging device may or may not be aligned with a desired image edge position on the photoreceptive member. In addition, there may be a misalignment created between the registration position or edge on the platen and such start-of-charging position of the charging device. The challenge to the user or customer then is to replace the charging device such that its start-of-charging position is precisely aligned with the registration position on the platen, and with a corresponding image edge position on the photoreceptive member. Failure to achieve such alignment ordinarily will result each time in an image on a copy sheet of paper that has a darkened border, or that has a clipped image portion adjacent such border, either of which is unacceptable.
There is therefore a need to provide for use in an electrostatographic reproduction machine, a customer replaceable unit that includes a self-aligning charging device which automatically aligns its start-of-charging position with a platen document registration position.